This invention generally relates to tube expansion and more particularly relates to an apparatus and method for expanding tubular members, such as heat transfer tubes and repair sleeves of the kind found in typical nuclear steam generators.
A typical nuclear steam generator or heat exchanger generates steam when heat is transferred from a heated and radioactive primary fluid to a non-radioactive secondary fluid of lower temperature. The primary fluid flows through a plurality of U-shaped tubes that pass through a plurality of support plates disposed in the steam generator. The ends of the tubes are received through holes in a tubesheet, which is also disposed in the steam generator. The ends of the tubes are in communication with a bowl-shaped lower plenum located below the tubesheet, the lower plenum being divided into an inlet plenum chamber and an outlet plenum chamber. During operation of the steam generator, the secondary fluid surrounds the exterior surfaces of the tubes as the primary fluid flows from the inlet plenum chamber, through the tubes and into the outlet plenum chamber. As the heated primary fluid flows through the tubes, the walls of the tubes function as heat conductors for transferring heat from the primary fluid to the secondary fluid. As the heat is transferred from the primary fluid to the secondary fluid, a portion of the secondary fluid vaporizes to steam for generating electricity in a manner well known in the art.
Occasionally, due to tube wall intergranular cracking caused by stress and corrosion during operation, the steam generator tubes may degrade (i.e., experience tube wall thinning) and thus may not remain leak-tight. If through-wall cracking occurs due to the degradation, the radioactive primary fluid may leak through the crack and commingle with the nonradioactive secondary fluid, a highly undesirable result.
However, if degradation is suspected, the tube, although degraded, may remain in service by sleeving the degraded portion of the tube. When sleeving is performed, a tubular repair sleeve is inserted into the tube to cover the degraded portion of the tube. The sleeve is then secured to the tube by radially expanding the sleeve into intimate engagement with the inner wall of the tube, such that the degraded portion of the tube is spanned or covered. In this manner, the radioactive primary fluid is prevented from commingling with the non-radioactive secondary fluid even though the wall of the tube is degraded. Such expansion of the sleeve is usually accomplished by means of a mechanical or hydraulic expansion mandrel.
Moreover, there is usually an annular gap defined between the outer walls of the tubes and the inner walls of the holes in the tubesheet through which the ends of the tubes are received. Potentially corrosive sludges (e.g., iron oxides, copper compounds and other metals), which settle-out of the secondary fluid, can accumulate on the upper surface of the tubesheet and flow down into such annular gaps. To prevent these potentially corrosive sludges from collecting within the annular gaps, each heat transfer tube is radially expanded to close the gap defined between the outer wall of the tube and the inner wall of the hole in the tubesheet. As in the previously mentioned case of sleeving, such expansion of the heat transfer tube for purposes of gap reduction is usually performed by means of a mechanical or hydraulic expansion mandrel.
However, applicant has observed that it is difficult to perform sleeving or tube-to-tubesheet gap reduction when the end of the heat transfer tube is located adjacent the curved sides of the bowl-shaped lower plenum of the steam generator. That is, the limited space available between the ends of the heat transfer tubes and the sides of the bowl-shaped lower plenum make it difficult to insert prior art mandrels into the tube ends to perform the required tube-to-tubesheet gap reduction and sleeving. This is so because prior art expansion mandrels are straight and rigid; thus, such prior art mandrels cannot be bent to maneuver them through such a confined space for insertion into the tube ends. Hence, repair of such tubes is difficult and sometimes impossible. Therefore, a problem in the art is to provide an expansion mandrel suitable for insertion into the ends of heat transfer tubes located adjacent the periphery of the tubesheet (i.e., adjacent the sides of the bowl-shaped lower plenum of the steam generator).
Also, it is sometimes necessary to repair a portion of the tube that is located at or beyond the tightly curved upper U-bend region of the tube. Therefore, it is preferable that such an expansion mandrel be capable of traversing the upper U-bend region of the heat transfer tube. However, prior art expansion mandrels are rigid and thus cannot readily bend to traverse the upper U-bend region of the heat transfer tube. Therefore, another problem in the art is o provide an expansion mandrel capable of traversing the U-bend upper region of the heat transfer tube.
In addition, applicant has observed that prior art expansion mandrels require lubrication for easier insertion into the heat transfer tube. Such lubrication is particularly needed for inserting the mandrel into tubes located in the confined space adjacent the sides of the bowl-shaped lower plenum the steam generator. However, such lubrication increases the time for completing the repair process because the lubricants, require extensive post-cleaning operations to avoid possible chemical reaction with the tube material during operation of the steam generator. Therefore, yet another problem in the art is to provide an expansion mandrel that does not require the use of lubricants.
Moreover, applicant has further observed that prior art expansion mandrels require a relatively close tolerance fit between the mandrel and the inside diameter of the tube or sleeve to provide the appropriate amount of outwardly directed force against the inside diameter. However, such a close tolerance increases the risk of frictional wear on undegraded portions of the heat transfer tube as the mandrel is inserted into the tube and translated therein. The risk of frictional wear is greatest when attempting to maneuver the mandrel into the tubes located adjacent the periphery of the tubesheet (i.e., adjacent the sides of the bowl-shaped lower plenum of the steam generator). Therefore, another problem in the art is to provide an expansion mandrel that reduces the risk of frictional wear on the tube.
Furthermore, applicant has observed that the usefulness of prior art expansion mandrels is also limited by the amount of diametrical expansion growth and tube ovality (i.e., the amount the tube is out-of-round). That is, mandrels having O-ring/urethane seals typically possess a maximum diametrical expansion capability of only approximately 0.045 inch with little or no tolerance for tube ovality due to the close tolerances of the metal-to-urethane interfaces. If the close tolerances of the metal-to-urethane interfaces are not adhered to, then the seals will tend to extrude until failure as they are subjected to high expansion pressures. In addition, expansion of the mandrel beyond approximately 0.045 inch may cause permanent or completely plastic extrusion of the mandrel, thereby requiring replacement of the mandrel. Such permanent extrusion of the mandrel may also cause difficulty in withdrawing the mandrel from the tube without damaging the tube. The difficulty of withdrawing such a permanently extruded mandrel from the heat transfer tube is greatest with regard to tubes located adjacent the sides of the bowl-shaped lower portion of the steam generator (i.e., adjacent the periphery of the tubesheet). Therefore, yet another problem in the art is to provide a tube expansion mandrel that is sized to expand without failure and that may be inserted into and withdrawn from a tube even though the tube is out-of-round (i.e., oval).
Expansion mandrels for expanding heat transfer tubes are known. One such mandrel is disclosed by U.S. Pat. No. 4,724,595 issued Feb. 16, 1988 in the name of David A. Snyder entitled "Bladder Mandrel For Hydraulic Expansions of Tubes And Sleeves" and assigned to the assignee of the present invention. However, the Snyder mandrel is straight and rigid. Thus, this patent does not appear to disclose an expansion mandrel suitable for insertion into the heat transfer tubes located adjacent the periphery of the tubesheet. In other words, this patent does not appear to disclose an expansion mandrel that does not require the use of lubricants, that reduces the risk of frictional wear on the tube, that is capable of traversing the upper U-bend region of the heat transfer tube, and that may be inserted into and withdrawn from the tube even though the tube is out-of-round.
Therefore, what is needed are an apparatus and method for suitably expanding tubular members, such as heat transfer tubes and repair sleeves of the kind found in typical nuclear steam generators.